Py file not get compiled for customization

Hello all,
I have some issue with custom coding via custom App. I need some function to added in some of the file of erpnext, so that i can call it from my custom App. but the py file in which I am writing my function is not getting complied. My instance is in development mode. Can anyone suggest me solution?

@Deep

Share what have you tried or what is your scenario. So it will easy to understand your concern.